A loudspeaker is a device which converts an electrical signal into an acoustical signal (i.e., sound) and directs the acoustical signal to one or more listeners. In general, a loudspeaker includes an electromagnetic transducer which receives and transforms the electrical signal into a mechanical vibration. The mechanical vibrations produce localized variations in pressure about the ambient atmospheric pressure; the pressure variations propagate within the atmospheric medium to form the acoustical signal. When the wavelength of a radiated acoustical signal is much larger than the physical dimensions of the device producing the signal, the radiation pattern tends toward omnidirectional. However, many applications require a device with a significant level of directivity. Typically, the target listening audience is localized in a particular region relative to the source, and an omnidirectional radiator directs the acoustical signal to regions other than the target region.
Even at low frequencies, a somewhat directional pattern may be obtained by utilizing two sources. If two sources are placed on a vertical axis separated by a distance D, the resulting acoustical signal will be completely nulled above and below the sources when D is .lambda./2, .lambda. being the wavelength of the acoustical signal radiated by the sources. The frequency corresponding to such a wavelength is referred to as the Maximum Off-Axis Rejection Frequency. Even when the wavelength of the signal varies from .lambda./2 by moderate amounts, a significant null remains above and below the radiators. When measured with typical 1/3 octave band resolution, such a configuration produces a minimum vertical beamwidth of 160 degrees over a 1/3 octave. The beamwidth of an acoustical system is defined as the angle that includes all of the acoustical output that is within 6 dB of the maximum output. The vertical beamwidth is the beamwidth within a vertical plane relative to the radiator.
An ideal loudspeaker would provide consistent radiation pattern control over the entire working frequency range. In a typical application, many loudspeakers will be incorporated into an array to provide sound to a wide listening area. If the radiation patterns of the loudspeakers within the array do not remain consistent with respect to frequency, particular listeners may be left out at some frequencies (as the beamwidths narrow) and particular listeners may be in an overlap region for some frequencies (as the beamwidths widen). An overlap may cause interference patterns to occur which distort the true acoustical signal. Thus, an inconsistent radiation pattern with respect to frequency makes it difficult to predictably array loudspeakers.
Considering the aforementioned limitations at low frequencies, a practical vertical beamwidth-verses-frequency goal is shown in FIG. 1. At the lowest working frequency, the vertical beamwidth is approximately 160 degrees. As the frequency increases, the beamwidth gradually narrows to the target middle/high frequency directivity (in this case approximately 35 degrees), at which point the curve flattens out, and the beamwidth remains relatively constant for increasing frequencies. One problem with realizing the directional characteristics of FIG. 1 is that a single driver normally cannot produce the entire desired frequency range, and therefore several drivers are often used to construct a loudspeaker system(i.e., two-way loudspeaker systems, three-way loudspeaker systems, etc.), where each driver is specifically designed to produce a particular frequency range. Crossover networks within the loudspeaker system receive the composite input signal, separate it into multiple frequency bands and provide a signal, representative of each frequency band, to each appropriate driver. The filters within the crossover network are not ideal, and so the frequency bands that the drivers receive overlap to some extent. Thus, the crossover frequency is a frequency within a crossover band. Since each driver is typically a unique design for a particular frequency band, each driver tends to have a unique beamwidth-verses-frequency characteristic, independent of the other drivers within the system. Consequently, a beamwidth discontinuity may occur at a crossover frequency, as shown in FIG. 2. Such a discontinuity causes the directional characteristics of the overall loudspeaker system to deviate from the ideal beamwidth-verses-frequency characteristic shown in FIG. 1.
It is an object of this invention to provide a loudspeaker system that substantially overcomes the aforementioned disadvantages.
It is another object of this invention to provide a loudspeaker system that exhibits a continuous and consistent beamwidth-verses-frequency characteristic over the entire working frequency range.
It is a further object of this invention to provide a loudspeaker system that exhibits continuous consistent directional pattern characteristics verses frequency, while occupying a relatively small amount of physical space.